In recent years, opportunities to use an electrophotographic copier and a printer have been increased in the field of printing as well as color printing. In the field of printing as well as color printing, high quality digital monochromatic or color images tend to be demanded. In order to respond to such a demand, it is proposed that a laser light having a short wavelength is employed as a source for exposure to light to form high definition digital images (refer to Patent Document 1).
However, the electrophotographic image quality finally obtained is not always a sufficiently high image quality, even though the laser light having a short wavelength is simply employed, and the dot size of exposure is narrowed to form a minute electrostatic latent image on the electrophotographic photoreceptor.
The reason is that photosensitive properties of the electrophotographic photoreceptor, an electrification characteristic of toner in a developer and so forth do not satisfy properties desired for formation of minute dot latent images as well as formation of toner images.
That is, in cases where the electrophotographic photoreceptor is an electrophotographic photoreceptor developed for a conventional long wavelength laser, (hereinafter, also referred to simply as a photoreceptor), reproducibility of dot images is not sufficient since a sensitivity characteristic is degraded, and no clear dot latent image is formed, when imagewise exposure in which the dot size of exposure is narrowed is conducted with laser light having a short wavelength.
On the other hand, as to an electrophotographic photoreceptor, a function-separation type electrophotographic photoreceptor in which a charge generation layer and a charge transport layer are layered in order to satisfy a photosensitive property, a mechanical property and so forth, that is, an organic photoreceptor in which an organic compound is used as charge generation layer (CGM) and charge transport layer (CTM) tends to be employed.
In this case, there appeared a problem such that the charge transport material for charge transport layer, having been developed for a conventional photoreceptor was easy to absorb light exhibiting a short wavelength of 300-500 nm, whereby degradation of electrophotographic characteristics such as drop in sensitivity drop, rise in residual potential, generation in transfer memory and so forth was produced.
In order to solve such a problem, a charge transport material having a triphenyl amine structure or the like, for example, which exhibits less absorption in the visible light range is known as a charge transport material for a photoreceptor for light exposure employing laser having a short wavelength (refer to Patent Documents 2, 3 and 4).
However, there appeared another problem such that neither sufficient sensitivity, nor sufficient durability was obtained even though a compound having such a triphenyl amine structure was employed as a charge transport material.    (Patent Document 1) Japanese Patent Open to Public Inspection (O.P.I.) Publication No. 2000-250239    (Patent Document 2) Japanese Patent O.P.I. Publication No. 63-278065    (Patent Document 3) Japanese Patent O.P.I. Publication No. 7-261424    (Patent Document 4) Japanese Patent O.P.I. Publication No. 2006-104183